1. Field of the Invention
This invention relates to a solid-state image pickup device for color pictures, and more particularly to a solid-state color imaging device in which the signal-to-noise ratios and the resolving powers for the respective primary colors are high.
2. Description of the Prior Art
Desiredly, a solid-state imaging device is should have the same extent of resolving power as that of an image pickup tube presently under use for television broadcasting. Even for the monochrome image pickup, an array of 500.times.500 photoelectric elements, switches for X-Y addressing these elements, and X- and Y-scanners which serve to turn the switches "on" and "off" and each of which consists of 500 stages are necessary. On the other hand, the chip size of a solid-state imaging device IC is limted from the aspects of manufacturing techniques and materials. Accordingly, a solid-state imaging device is usually fabricated through silicon MOS IC technology which can achieve a high integration density. In the case of the MOS structure, the source junction of a MOS field effect transistor (hereinafter, abbreviated to "MOST"), which is used as a switch at the time when charges stored in the photoelectric device are transmitted as a picture signal in response to a scanning pulse, can be utilized as a photodiode. This brings forth the great advantage that the photoelectric device and the switch can be fabricated integrally. In addition, the yield of fabrication is high. A photoelectric device employing silicon as the parent material, however, exhibits a low sensitivity in visible light and has the following serious problems as the photoelectric device for color image pickup:
(a) The optical absorption edge of silicon lies at 9,000 A in terms of wavelength, and the sensitivity thereof in the infrared region is high. However, the photosensitivity of silicon in the visible region (4,500 A-6,600 A), especially the blue region (4,500 A), necessary for the ordinary image pickup is conspicuously low. The present inventors fabricated for test purposes a three-plate type color camera which employed three solid-state imaging devices of the silicon MOS structure for the colors blue (4,500 A), green (5,500 A) and red (6,600 A), respectively. As a result, it was revealed that the sensitivity of the imaging device for blue light is about 1/5 of the sensitivity of the highly sensitive imaging device for red light and that the sensitivity of the imaging device for green light is about 2/3 of the sensitivity of the imaging device for red light. Charges created by light are stored in the junction capacitances of the photodiodes, and are read out at a predetermined period (30 Hz in the NTSC system). Where charges for charging up the capacitance are stored in the photodiode for red light, signal charges are stored in an amount of only 2/3 of the capacitance in the photodiode for green light and in an amount of only 1/5 of the capacitance in the photodiode for blue light. Accordingly, the signal-to-noise ratio of the device for blue light is very low and sometimes becomes a serious hindrance to practical use. On the other hand, when the enhancement of the signal-to-noise ratio is intended in such a way that signal charges charge up the diode capacitance in the device for blue light, charges corresponding to four times the diode capacitance cannot be accommodated in the diode capacitance in the device for red light and double the charges cannot be accomodated in the diode capacitance in the device for green light. Therefore, they overflow to the surrounding material, and degradations of the resolving powers are incurred in the devices for red light and for blue light.
(b) Light of longer wavelengths than red light, i.e., infrared radiation, also enters to create unnecessary charges. Since the absorption coefficient of silicon for infrared radiation is small, infrared radiation penetrates deep into the silicon substrate. The charges generated there diffuse, not only into the diode at the position of incidence, but also into the surrounding diodes, which causes a lowering of the resolving power as stated at the end of Item (a).
In the case where the separate imaging devices for the respective three primary colors are employed as described above, an optical system needs to be separated to some extent for the respective three primary colors. This renders the apparatus large-sized. In addition, a difficult operation of aligning the three imaging devices is required in order to exactly register images of the three primary colors, and the whole camera inevitably becomes expensive.